1. Field of the Invention
The invention relates generally to magnetic suspensions for pointing instruments on a levitated member, movable in rotation about an axis or in linear translation. More particularly, the invention pertains to an electronic circuit configuration for determining the position of a suspension member with respect to associated electromagnetic actuators, for use in linearizing the applied deflection force.
2. Description of the Prior Art
Magnetic suspensions are particularly well adapted for substantially frictionless suspension of a movable member. Prior suspensions of this type utilizing magnetic bearing assemblies, in which it is desired to provide an output force linearly proportional to a commanded input force, require compensation for the nonlinear nature of the magnetic circuit, since the force obtained is a nonlinear function of coil current and the air gap between the magnetizing coils and the moveable member. The resulting magnetic force F has been found to be of the form EQU F=KI.sup.2 /g.sup.2 ( 1)
where I is the current applied to an actuator coil, g is the air gap between a magnetically permeable core of the coil and the magnetic suspension member and K is a factor of proportionality. Since the magnetic actuator typically consists of two essentially identical magnetic circuits applying oppositely directed forces to a common rotor or armature, it may be shown that the square law current characteristic can be compensated by applying a fixed bias to the variable control current supplied to each coil. By sensing the position of the armature in the magnetic gap, the currents may be further modulated to negate the effects of the reciprocal gap-squared factor.
Thus prior suspension technology has required accurate measurements of both applied current and air gap between the armature and the coil. While precise current measurements are readily obtained, making an air gap measurement with the requisite linearity and resolution requires a precision sensor which is costly to fabricate and may result in reduced system reliability. A further disadvantage of the aforementioned apparatus is that equation (1) does not allow for the effects of magnetic hysteresis, flux leakage across the gap, and other magnetic anomalies. In consequence, magnetic actuators controlled in this way have at best achieved of the order of one or two percent linearity, even under controlled conditions. In Magnetic Suspension: The Next Generation in Precision Pointing, American Astronautical Society, Paper No. AAS 82-034, Jan. 30, 1982, the present inventor discussed an apparatus for correcting for such errors. By measuring the actual force applied to the magnetic bearing, as well as the magnitude of the air gap, and feeding the actual force signal in a closed loop to correct the force command signal, there is obtained improved performance with linearity of the order of one-tenth of one percent and compensation for the effects of the magnetic anomalies. However achieving an accurate air gap measurement requires a precision proximity transducer (proximeter). The present invention obviates the need for a precise measurement of the armature position in the gap when used in a force-loop system.